In known gas turbines—both aircraft engines and stationary gas turbines for generation of electricity—the fuel is more or less completely converted into combustion products, mainly CO2 and H2O, within the combustion chamber. In a guide vane row, downstream of the combustion chamber, the exhaust gases are accelerated and deflected towards at least one blade row to perform mechanical work. Usually a couple of vane and blade row stages are subsequently arranged to exploit the kinetic and thermal energy of the hot gas flow.
The maximum temperature reached in the combustion chamber and the residence time of the combustion products in the combustion chamber dictate the undesired production of NOx and CO. Less important parameters for the generation of nitrogen oxides are the pressure in the combustion chamber and the fuel composition.
It is known per se to achieve a substantial local reduction of the maximum combustion temperature and the residence time at this temperature and thus the formation of undesired nitrogen oxides by improving the quality of mixing of fuel and air, including homogenization of fuel and air mixture within a premixing burner or swirl generator elements or between individual burners or swirl generator elements, or by reducing the leakage of air and heat loss of the combustion chamber or by reducing the residence time of the hot combustion gases in the combustion chamber. Such improvement measures become increasingly challenging and the achievable NOx reductions become smaller.
The reduction of NOx-formation through a reduction in the residence time is limited, because CO-emissions increase at part load.
One way for reducing the combustion temperature while maintaining the same rotor inlet temperature is to cool the turbine vanes with a closed cooling circuit, using steam as a coolant.
Disadvantages of this method are the complexity of an additional cooling management, the additional costs and a to a certain degree lower reliability and availability of the system.
WO 2006/053825 and DE 10043933 describe a combustor to achieve reduced pollutant emissions, particularly NOx, by a first combustion chamber with a relatively low residence time with the consequence of an incomplete burnout, a first vane row downstream of the first combustion chamber, and following a second combustion zone, a sequential combustion chamber, with or without injection of additional fuel and with sufficient residence time for complete burnout. Said first vane row aligns the flow of the hot combustion gases in axial direction. In addition, the gas flow is accelerated with the consequence of a significant reduction of its temperature. Accordingly, the temperature in the second combustion zone is lower than in the first combustion chamber. This lower temperature level is matched with the parameters of NOx-formation. In this way, a significant reduction of NOx-formation is reached.
WO 2011/061059 relates to a reheat combustor for a gas turbine. This document discloses features, how to cool the first and the sequential combustion chamber by cooling air, which is subsequently supplied to the combustion chamber and premixed with fuel. The goal is to reduce the difference between combustor exit temperature and the maximum temperature in the premixed flame to a minimum, thus reducing the strongly temperature dependent NOx-formation.